The technological development of nanomaterials has been underway for decades. Only in the past decade has serious attention been paid to their potential unwanted health and environmental effects. Many nanomaterials do not readily dissolve or disintegrate in biological and environmental milieu. As a result, they can persist where they are sequestered (in mammals, mostly in mononuclear phagocyte system organs such as the liver, spleen and bone marrow) for months, possibly years, often associated with inflammatory/oxidative stress responses, including granuloma. These unwanted effects have been seen with nanoceria (nanoscale cerium dioxide). On the other hand, nanoceria has been shown to be an effective anti-inflammatory/antioxidant in numerous cellular, cell, and whole animal models of inflammation/oxidative stress. Nanoceria's pro- and antioxidant effects result from its auto-catalytic redox behavior (oxidation from Ce (III) to Ce (IV) and reduction back to Ce (III)). The first specific aim of this project will determine if nanoceria can produce inflammatory/pro- oxidant effects in the absence of elevated inflammation/oxidative stress and anti-inflammatory/antioxidant effects when inflammation/oxidative stress is elevated, and determine the lowest observed adverse dose and lowest observed beneficial dose. Four nanoceria will be studied: a 5 and a 30 nm nanoceria extensively studied for their distribution, persistence and effects in the rat (preliminary findings) and a third that others have demonstrated produces beneficial inflammatory/pro-oxidant effects. These three will be synthesized and extensively characterized in-house. The fourth will be NM-212, a commercial nanoceria that is being extensively studied by the inhalation route by others. Due to the very low oral and pulmonary absorption of nanomaterials, nanoceria will be delivered by intravenous injection to establish sufficient levels in multiple organs to study its biodistribution, persistene, biotransformation, and effects. Preliminary findings indicate that nanoceria undergoes some bioprocessing in mammalian cells over months to a more stable form. This appears to occur via dissolution and formation of very small nanoceria particles. These very small nanoceria particles have a greater enrichment of Ce (III) on their surface; therefore they are expected to have enhanced anti-inflammatory/antioxidant properties. This biotransformation suggests enhanced benefit over time associated with nanoceria's persistence and bioprocessing. The second specific aim will test the hypothesis that the distribution, form, and effects of nanoceria change over time due to its bioprocessing in the liver to smaller, more antioxidant forms. The third specific aim will identify factors that contribute to nanoceria dissolution and precipitation and mediate its bioprocessing and precipitation in the liver. The proposed studies will identify nanoceria doses that maximize its efficacy relative to its unwanted effects, elucidate nanoceria's long term fate in a model mammal, the rat, and provide the insight into its biotransformation that may enable safer by design nanoceria for use as a therapeutic agent with prolonged anti-inflammatory/antioxidant activity.